1. Field of the Invention
This invention relates to a sensitive relay of the polarized type wherein the thresholds thereof are precisely defined, regardless of the characteristics of the relay.
2. Description of the Prior Art
It is known that all electromechanical relays have rather imprecise closing and opening thresholds, which render their application difficult in threshold-type functions, i.e., those in which their operation is to be controlled for a defined value of control current or voltage, especially if these vary slowly.
More particularly, in sensitive relays of the polarized type, this mechanical instability is compounded by magnetic instability.
One particularly interesting type of polarized sensitive relay is the Hughes relay with all its numerous described variants (e.g., French Pat. No. 2,087,977). It consists of a relay in which the movable armature is closed when at rest and held closed (against an opposing force) by means of the permanent flux created by a magnet or solenoid. An electrical pulse, injected into a coil around the stationary or movable armature of the relay, momentarily cancels this permanent flux and the armature opens.
In principle, the electrical release threshold (i.e., the pulse amplitude) is perfectly defined; but in practice, even here, the instability of polarization, of the air gap, and of the armature return force, as well as shocks, vibrations, etc., make the opening threshold of this relay unreliable. This relay has the advantage of being very sensitive and of being able to operate by direct or alternating current. In the latter case, the alternation of the correct polarity comes into play, at least as long as its duration is greater than or equal to the response time of the relay, i.e., the time for the movable armature to move far enough away from the stationary armature for the movement to become irreversible. An additional drawback of this relay appears when the control voltage pulse becomes clearly greater than the threshold value. Here the electrical excitation (amperes.times.revolutions) in opposition to the permanent polarization of the relay will not only "depolarize" the relay (the case of the permanent polarization magnet) but will also eliminate all release (here the relay holds by inverse magnetic induction).
For example, in the case of a sinusoidal control current, if the electrical angle at which excitation passes into the flux reduction zone (encouraging release) becomes small, i.e., less than the time required for the movable armature to reach the point of no return, the relay no longer responds. This is obviously unacceptable in all electrical protection applications, e.g., where even large overloads must open the relay so as to protect a facility.
Some of the cited drawbacks were eliminated in a recent concept (e.g., French Pat. Nos. 1,323,673; 1,358,355; 1,407,271; 1,411,747; and the patent cited above) in which the operating threshold of the relay was determined solely by the threshold of a four-layer diode type electronic switch (e.g., a Shockley diode) or equivalent circuits (U.S. Pat. No. 2,655,609).
However, the very principle of these electronic threshold switches does not enable precise thresholds to be obtained, except to the extent that the as yet unexcited currents delivered by the signal source are sufficiently strong. This limits the maximum sensitivity which may be practically attained, as well as the sensitivity of the threshold, if the source is of the microenergy type, i.e., the high internal impedance type.
Finally, in the cited electronic threshold relays, applied to the principle of amplification by accumulation, the "microenergy" accumulated progressively in a storage element (such as a capacitor) is discharged through the electronic threshold switch into the opening or closing relays. The very principle of this discharge (charged capacitor delivering onto an essentially inductive charge) translates into poor energy adaptation since sinusoidal current, with relatively slow set-up of the excitation current, is unfavorable.